Index: head/lib/libthr/libthr.3 =================================================================== --- head/lib/libthr/libthr.3 (revision 272069) +++ head/lib/libthr/libthr.3 (revision 272070) @@ -1,62 +1,275 @@ .\" Copyright (c) 2005 Robert N. M. Watson +.\" Copyright (c) 2014 The FreeBSD Foundation, Inc. .\" All rights reserved. .\" +.\" Part of this documentation was written by +.\" Konstantin Belousov under sponsorship +.\" from the FreeBSD Foundation. +.\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD$ .\" -.Dd October 19, 2007 +.Dd September 20, 2014 .Dt LIBTHR 3 .Os .Sh NAME .Nm libthr .Nd "1:1 POSIX threads library" .Sh LIBRARY .Lb libthr .Sh SYNOPSIS .In pthread.h .Sh DESCRIPTION The .Nm library provides a 1:1 implementation of the .Xr pthread 3 library interfaces for application threading. It has been optimized for use by applications expecting system scope thread semantics, and can provide significant performance improvements compared to .Lb libkse . +.Pp +The library is tightly integrated with the run-time link editor +.Xr ld-elf.so.1 1 +and +.Lb libc ; +all three components must be built from the same source tree. +Mixing +.Li libc +and +.Nm +libraries from different versions of +.Fx +is not supported. +The run-time linker +.Xr ld-elf.so.1 1 +has some code to ensure backward-compatibility with older versions of +.Nm . +.Pp +The man page documents the quirks and tunables of the +.Nm . +When linking with +.Li -lpthread , +the run-time dependency +.Li libthr.so.3 +is recorded in the produced object. +.Sh MUTEX ACQUISITION +A locked mutex (see +.Xr pthread_mutex_lock 3 ) +is represented by a volatile variable of type +.Dv lwpid_t , +which records the global system identifier of the thread +owning the lock. +.Nm +performs a contested mutex acquisition in three stages, each of which +is more resource-consuming than the previous. +.Pp +First, a spin loop +is performed, where the library attempts to acquire the lock by +.Xr atomic 9 +operations. +The loop count is controlled by the +.Ev LIBPTHREAD_SPINLOOPS +environment variable, with a default value of 2000. +.Pp +If the spin loop +was unable to acquire the mutex, a yield loop +is executed, performing the same +.Xr atomic 9 +acquisition attempts as the spin loop, +but each attempt is followed by a yield of the CPU time +of the thread using the +.Xr sched_yield 2 +syscall. +By default, the yield loop +is not executed. +This is controlled by the +.Ev LIBPTHREAD_YIELDLOOPS +environment variable. +.Pp +If both the spin and yield loops +failed to acquire the lock, the thread is taken off the CPU and +put to sleep in the kernel with the +.Xr umtx 2 +syscall. +The kernel wakes up a thread and hands the ownership of the lock to +the woken thread when the lock becomes available. +.Sh THREAD STACKS +Each thread is provided with a private user-mode stack area +used by the C runtime. +The size of the main (initial) thread stack is set by the kernel, and is +controlled by the +.Dv RLIMIT_STACK +process resource limit (see +.Xr getrlimit 2 ) . +.Pp +By default, the main thread's stack size is equal to the value of +.Dv RLIMIT_STACK +for the process. +If the +.Ev LIBPTHREAD_SPLITSTACK_MAIN +environment variable is present in the process environment +(its value does not matter), +the main thread's stack is reduced to 4MB on 64bit architectures, and to +2MB on 32bit architectures, when the threading library is initialized. +The rest of the address space area which has been reserved by the +kernel for the initial process stack is used for non-initial thread stacks +in this case. +The presence of the +.Ev LIBPTHREAD_BIGSTACK_MAIN +environment variable overrides +.Ev LIBPTHREAD_SPLITSTACK_MAIN ; +it is kept for backward-compatibility. +.Pp +The size of stacks for threads created by the process at run-time +with the +.Xr pthread_create 3 +call is controlled by thread attributes: see +.Xr pthread_attr 3 , +in particular, the +.Xr pthread_attr_setstacksize 3 , +.Xr pthread_attr_setguardsize 3 +and +.Xr pthread_attr_setstackaddr 3 +functions. +If no attributes for the thread stack size are specified, the default +non-initial thread stack size is 2MB for 64bit architectures, and 1MB +for 32bit architectures. +.Sh RUN-TIME SETTINGS +The following environment variables are recognized by +.Nm +and adjust the operation of the library at run-time: +.Bl -tag -width LIBPTHREAD_SPLITSTACK_MAIN +.It Ev LIBPTHREAD_BIGSTACK_MAIN +Disables the reduction of the initial thread stack enabled by +.Ev LIBPTHREAD_SPLITSTACK_MAIN . +.It Ev LIBPTHREAD_SPLITSTACK_MAIN +Causes a reduction of the initial thread stack, as described in the +section +.Sx THREAD STACKS . +This was the default behaviour of +.Nm +before +.Fx 11.0 . +.It Ev LIBPTHREAD_SPINLOOPS +The integer value of the variable overrides the default count of +iterations in the +.Li spin loop +of the mutex acquisition. +The default count is 2000, set by the +.Dv MUTEX_ADAPTIVE_SPINS +constant in the +.Nm +sources. +.It Ev LIBPTHREAD_YIELDLOOPS +A non-zero integer value enables the yield loop +in the process of the mutex acquisition. +The value is the count of loop operations. +.It Ev LIBPTHREAD_QUEUE_FIFO +The integer value of the variable specifies how often blocked +threads are inserted at the head of the sleep queue, instead of its tail. +Bigger values reduce the frequency of the FIFO discipline. +The value must be between 0 and 255. +.El +.Sh INTERACTION WITH RUN-TIME LINKER +The +.Nm +library must appear before +.Li libc +in the global order of depended objects. +.Pp +Loading +.Nm +with the +.Xr dlopen 3 +call in the process after the program binary is activated +is not supported, and causes miscellaneous and hard-to-diagnose misbehaviour. +This is due to +.Nm +interposing several important +.Li libc +symbols to provide thread-safe services. +In particular, +.Dv errno +and the locking stubs from +.Li libc +are affected. +This requirement is currently not enforced. +.Pp +If the program loads any modules at run-time, and those modules may require +threading services, the main program binary must be linked with +.Li libpthread , +even if it does not require any services from the library. +.Pp +.Nm +cannot be unloaded; the +.Xr dlclose 3 +function does not perform any action when called with a handle for +.Nm . +One of the reasons is that the interposing of +.Li libc +functions cannot be undone. +.Sh SIGNALS +The implementation also interposes the user-installed +.Xr signal 3 +handlers. +This interposing is done to postpone signal delivery to threads which +entered (libthr-internal) critical sections, where the calling +of the user-provided signal handler is unsafe. +An example of such a situation is owning the internal library lock. +When a signal is delivered while the signal handler cannot be safely +called, the call is postponed and performed until after the exit from +the critical section. +This should be taken into account when interpreting +.Xr ktrace 1 +logs. .Sh SEE ALSO -.Xr pthread 3 +.Xr ktrace 1 , +.Xr ld-elf.so.1 1 , +.Xr getrlimit 2 , +.Xr umtx 2 , +.Xr dlclose 3 , +.Xr dlopen 3 , +.Xr errno 3 , +.Xr getenv 3 , +.Xr libc 3 , +.Xr pthread_attr 3 , +.Xr pthread_attr_setstacksize 3 , +.Xr pthread_create 3 , +.Xr signal 3 , +.Xr atomic 9 .Sh AUTHORS .An -nosplit The .Nm library was originally created by .An Jeff Roberson Aq Mt jeff@FreeBSD.org , and enhanced by .An Jonathan Mini Aq Mt mini@FreeBSD.org and .An Mike Makonnen Aq Mt mtm@FreeBSD.org . It has been substantially rewritten and optimized by .An David Xu Aq Mt davidxu@FreeBSD.org .